The present invention relates generally to a drill bit and a method of surface decarburizing same and more particularly to a cemented carbide microdrill having refined primary cutting edges and a method of surface decarburizing same.
Microdrills generally range in diameter from about 0.002 inches to 0.125 inches. Microdrills are typically employed to form holes in single sided, double sided, and multilayer printed circuit board laminates. Microdrills may additionally be employed in any application where small holes of carefully controlled dimensions are required. Such additional applications include cameras, watches, fuel injectors and the like.
High quality holes of precise dimensions are required in printed circuit boards and other applications. This requires producing holes with minimal defects such as rough hole walls and burrs. These undesirable defects may be caused by chipped or worn drill bits. These problems and the accompanying need to reduce chipping and wear have been recognized previously. See U.S. Pat. No. 4,759,667 issued to Brown, and U.S. Pat. No. 4,561,813 issued to Schneider.
Microdrills for drilling holes in printed circuit boards must have sufficient strength and resistance to wear to satisfy the requirements of the industry. Accordingly, it is common to make such microdrills of cemented carbide to provide the strength and hardness required. A disadvantage of using cemented carbide, however, is its brittleness which causes such microdrills to be prone to chipping. This is especially so at the outer corner of the cutting edge of the drill bit which defines the wall of the hole drilled in the printed circuit board. This proneness to chipping of cemented carbide drills has been recognized previously. See U.S. Pat. No. 4,583,888 issued to Mori, et al., and U.S. Pat. No. 4,642,003 issued to Yoshimura.
To improve drill bit life, conventional processes have focused on heat treating the entire tool (surface and interior portion), rather than the surface. Others have also heat treated or annealed the sintered powder used in forming the drill bit rather than the finished tool in an attempt to extend drill bit life.
Attempts at improving resistance to wear and chipping of cemented carbide microdrills for printed circuit boards and other drills have been tried. Yet a superior microdrill of cemented carbide which is resistant to wear and chipping and which can be economically mass produced has not emerged.